Coal-fired power generation plants, municipal waste incinerators, and oil refinery plants generate large amounts of flue gases that contain substantial varieties and quantities of environmental pollutants, such as sulfur oxides (SO2, and SO3), nitrogen oxides (NO, NO2), mercury (Hg) vapor, and particulate matters (PM). In the United States, burning coal alone generates about 27 million tons of SO2 and 45 tons of Hg each year.
Regulations limiting emissions of mercury from coal-fired power plants are scheduled to take effect in the United States in 2016. Presently there are two commonly accepted methods for compliance with these regulations. The first involves addition of a brominated salt, such as calcium bromide to the pulverized coal. Upon combustion, bromine combines with mercury to produce a volatile mercuric bromide. Mercuric bromide is readily soluble in water and is efficiently captured in downstream scrubbers provided for flue gas desulfurization. Alternatively, a second method involves active carbon powder continuously injected into the flue gas. Activated carbons are reasonably effective in capturing elemental mercury. Injection typically occurs immediately upstream of unit operations designed to capture fly ash. Such operations may consist of an electrostatic precipitator or a fabric filter. Carbons injected to control elemental mercury emissions are also captured in the electrostatic precipitator or the fabric filter.
There are problems associated with existing methods of mercury control. In the case of the addition of brominated salts to the coal, bromine is produced in excess. Damage due to corrosion by bromine and hydrogen bromide is a risk that accompanies the use of this method. In the case of injection of powdered activated carbon, control of high levels of mercury may require large injection rates. This is especially true where high sulfur coal is combusted and there is a high SOx content in the flue gas. Further, injected carbon is captured with the fly ash, and high levels of injection may compromise the value of the fly ash, for example for sale into concrete.
It would be desirable to have a system and method of removing elemental mercury, sulfur oxides and other contaminants, which does not suffer from the drawbacks of the current methods, which introduce bromine into the hot flue gas, and which require large quantities of consumable carbon.
U.S. Pat. No. 6,132,692, discloses a process for reducing multiple pollutants (particles, Hg, NOx, and SO2) whereby an electrical barrier discharge reactor produces HgO and acids HNO3 and H2SO4, and a wet electrostatic precipitator (ESP) collects the HgO, acids, and particulates. The collected pollutants are then drained from the wet ESP for further processing. However, the SO2 and NOx removal efficiencies of this process are limited, the system is expensive, energy input is very high, and the collected acid solution may need treatment as liquid waste.
Use of a fixed bed adsorbent would seem to provide an attractive alternative. However, the use of fixed beds has heretofore been limited, due primarily to short adsorbent life. A fixed bed may need to operate without maintenance for a period of 1-3 years. In practical applications the life of a carbon adsorbent is typically too short to provide the necessary lifetime. Carbons can be treated with a variety of chemicals to improve the overall capacity. However, the actual lifetime is often limited by the accumulation of acid caused by oxidation of sulfur dioxide in the flue gas by activated carbon.
In order to overcome the effect of acid accumulation due to SO2 oxidation, Lu et al. (U.S. Pat. No. 7,442,352 B2, hereinafter '352 patent) proposed use of sorbent polymer composites, where activated carbon is combined in a hydrophobic polytetrafluoroethylene (PTFE) matrix, which acts as a “reverse sponge”, expelling acid as it is formed. The use of this novel sorbent polymer composite provides additional benefits. It can be fabricated into honeycomb shapes to provide highly efficient mercury capture with lower pressure drops than can be obtained through packed, granular beds of carbon. In order to increase the capacity of sorbent polymer composites a variety of halogen containing salts were used.
U.S. Pat. No. 8,524,186 B2 describes a carbon-based catalyst for flue gas desulfurization and method of producing the same and use thereof for removing mercury in flue gas. Limitations exist with this system to provide the levels of free iodine & bromine needed to treat the continuous stream of mercury. The iodine and bromide disclosed in the prior art are leached away in the processes discussed above.
In order to preserve the long term effectiveness of sorbent polymer composites, it would be desirable to have a carbon formulation which contained halogen salts capable of improving the effectiveness of mercury capture, but which are not easily lost through leaching.
There is a need to provide an improved durable system that can simultaneously remove multiple flue gas pollutants such as SOx, Hg vapor, and PM2.5 with low cost. It is desirable that the system is simple, does not generate secondary pollutants, and has the capability of producing a useful end product. In particular it would be ideal to develop a system that can provide a source of halogen (iodine and bromine compounds) in the required amount for a prolonged period of time. More specifically, a more durable and longer lasting halogen source in combination with a sorbent polymer composite substrate is desirable that does not get leached away in solutions developed in the treatment process.